In the formation of continuous glass fiber strands, glass filaments are typically attenuated through bushing tips or orifices located at the bottom of a heated bushing having molten glass therein. The filaments are attenuated through the bushing tips at speeds from about 1,000 to 15,000 feet per minute (304.8 to 4,572.0 meters per minute) or more. The filaments are drawn across the application surface of an applicator where they are coated with a binder and/or size to prevent abrasion and to add desired properties to the filaments. The filaments are then gathered into a unified strand in a gathering shoe, which is typically a grooved cylinder or wheel formed of material such as graphite, and are wound on a rotating collet as a forming package, with the rotation of the collet providing the attenuative forces necessary to form the filaments.
Glass filaments may range from about 0.0070 inch (0.0178 centimeter) and larger to about 0.00018 inch (0.0004572 centimeter) and smaller. These very small filaments can sustain only a very small tensile force before breaking and the breaking of a single filament among the hundreds or even thousands of filaments being drawn from a bushing requires an interruption of the forming process which reduces productivity and increases manufacturing costs.
Forces which break filaments in the forming process can originate from nonuniform thermal conditions in the space immediately below the bushing where the molten glass streams are attenuated and cooled. An increase in the rate of heat removal from an attenuating molten glass stream will increase its viscosity faster than desired, adding tension to the filaments being formed. If this added tension becomes sufficient to cause an increase in stress to the ultimate stress of the glass, the filament breaks. Changes in the air velocity or air temperature near the molten streams can change the viscosity and tension sufficiently to break the filaments.
The hundreds or even thousands of filaments being drawn downwardly from the bushing at speeds up to 15,000 feet per minute (4,572 meters per minute) or more drag surrounding air downwardly with them. The air being dragged downward by the speeding filaments is replaced by air from the immediate vicinity of the bushing, and it is not uncommon for the air to be sucked away from the forming cone directly below the bushing tips to satisfy the need below. When this occurs, the forming cone space draws air from its surroundings to replace the air stolen by the filaments. If this replacement air in the forming cone zone is erratic in either velocity or temperature, the stage is set for an interruption in production due to filament breakout based upon a change in air velocity and/or air temperature.
Just as a temporary excess of airflow below the bushing can cause filament breakage from high tension resulting from too rapid cooling of the molten glass forming cone, a temporary deficiency of airflow below the bushing can result in fiber disruption due to insufficient cooling of the forming cone and consequent separation of the glass stream as a result of the pinching forces of surface tension.
It is desirable, therefore, to reduce or eliminate erratic airflow and air temperature immediately below the bushing tips and thus to provide a more uniform airflow and uniform temperature in the region below the bushing.
It is known to attenuate discontinuous glass fibers by means of high velocity downward gas or steam jets. Typical of this attenuation are the methods shown in U.S. Pat. No. 2,224,466; 2,234,986; 3,021,558; 3,532,479; 3,547,610; 3,836,346 and 3,881,903. The velocities of the gas jets employed to attenuate the discontinuous fibers typically ranges from about 150 to 1700 feet per second (45.7 to 518.2 meters per second).
While these high gas velocities may be employed in the production of discontinuous glass fibers, such high gas velocities cannot be tolerated in the production of continuous glass fibers. These high velocities disrupt the operation of the bushing, due to erratic turbulent flow and thus erratic airflow and temperatures below the bushing, resulting again in discontinuous filaments. Thus, it is a further objective of the present invention to control the environment below a continuous glass fiber forming bushing with gas of a volume and velocity sufficient to produce uniform airflow and temperatures below the bushing but insufficient to attenuate the filaments or produce turbulent airflow below the bushing.